Pigments used in printing inks tend to be attracted to one another in the ink. If enough pigment particles become associated, the particles tend to fall out of solution. To prevent this, dispersants are added to the ink containing the pigment particles.
Dispersants generally use one of two methods for preventing the pigment particles from agglomerating. One method uses bulky groups attached to the pigment particle to prevent (or at least hinder) the pigment particles from being able to interact in such a way that they can coagulate. This method may be referred to as the steric hindrance method.
Another method which may be used to prevent the pigment particles from coagulating is electrostatic stabilization. This method generally associates dispersants with the pigment particles, the dispersants having a charged group which has the same charge as the other dispersants. This causes the individual dispersion complexes (pigment+dispersant) to be repelled from each other. Thus, two pigment particles each having a negatively charged dispersant associated with it will be repelled from each other due to the fact that like charges are repelled from each other.
Dispersants tend to be selected such that they sufficiently interact with the pigment particle to form a stabilized dispersion complex. While uncommon, it is also possible for a pigment particle to have sufficient steric hindrance to be sterically stabilized or sufficient charge to be electrostatically stabilized without the use of a dispersant. As used herein, dispersion will refer to any particle or complex of particles that tend to disperse from each other. Reference to a dispersion complex is a reference to the combination of a pigment and a dispersant which are associated and which complex tends to be dispersed from other dispersions.
With electrostatically stabilized pigment dispersions, excursions in pH can lead to their instability. With present pigmented inks, pH decreases with accelerated storage testing time, which can amount to 0.5 pH units. The decrease in pH also heralds an increase in ionic strength, which has an adverse, shrinking effect on the Debye screening length as well as protonization of the carboxylate groups on the dispersant. Thus, a double-barreled threat presents itself to a pigment's stability with shifts in pH over time.
Buffers may be employed to stabilize the pH, but, in order to achieve the buffering capacity to withstand pH drift, a high concentration of buffer is used which contributes significantly to the total ionic strength of the solution and thus lowers the Debye screening length as well.
The teachings herein below extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above-mentioned needs, benefits, or preferences.